1. Field of the Invention
The present invention relates to radiographic image conversion panels used in the formation of radiographic images.
2. Description of the Related Art
Radiographic images such as X-ray images have been widely used in medical diagnosis of disease conditions. In particular, radiographic images based on intensifying screen-film combinations have undergone enhancements in terms of sensitivity and image quality during a long history and consequently remain in use in the medical field worldwide as the imaging system with high reliability and excellent cost performance. However, this image information is analogue and thus cannot be processed freely or transmitted instantaneously in contrast to currently developing digital image information.
Recently, digital radiographic image detectors such as computed radiography (CR) systems and flat panel detectors (FPDs) have come in use. These radiographic image detectors directly give digital radiographic images and allow the images to be directly displayed on displays such as cathode ray tube panels and liquid crystal panels. Thus, there is no need for the images to be created on photographic films. Consequently, the digital radiographic image detectors such as X-ray image detectors have decreased a need for the image formation by silver halide photography and have significantly enhanced diagnostic convenience at hospitals and clinics.
The computed radiography (CR) is one of the digital X-ray image techniques currently used in medical practice. However, CR X-ray images are less sharp and are insufficient in spatial resolution as compared to screen film system images such as by silver halide photography, and the level of their image quality compares unfavorably to the quality level of screen film system images. Thus, new digital X-ray image techniques, for example, flat panel detectors (FPDs) involving thin film transistors (TFTs) have been developed (see, for example, Non Patent Literatures 1 and 2).
In principle, a FPD converts X-rays into visible light. For this purpose, a scintillator panel is used which has a phosphor (scintillator) layer made of an X-ray phosphor that, when illuminated with X-rays, converts the radiations into visible light that is emitted. In X-ray photography using a low-dose X-ray source, it is necessary to use a scintillator panel with high luminous efficiency (X-ray to visible light conversion) in order to enhance the ratio (the SN ratio) of signal to noise detected from the scintillator panel. In general, the luminous efficiency of scintillator panels is determined by the thickness of the scintillator layer (the phosphor layer) and the X-ray absorption coefficient of the phosphor. The light produced in the phosphor layer upon illumination with X-rays is scattered more markedly in the scintillator layer with increasing thickness of the phosphor layer, and consequently the sharpness of X-ray images obtained via the scintillator panel is lowered. Thus, setting of the sharpness required for the quality of X-ray images automatically determines the critical thickness of the phosphor layer in the scintillator panel.
Further, the shape of phosphor forming a phosphor layer is also important in order to obtain a scintillator panel which is able to give X-ray images having high brightness and excellent sharpness. In many scintillator panels, a scintillator layer is composed of columnar phosphor crystal. Usually, a plurality of such columnar crystals are disposed on bases such as substrates or supports. In order for scintillator layers to be able to efficiently emit light (fluorescence) produced therein in a direction perpendicular to the principal surface of the bases such as substrates or supports, the columnar crystals constituting the scintillator layers extend perpendicularly to the principal surface of the bases such as substrates or supports. With this configuration of scintillator layers, the scintillator panels ensure brightness and sharpness as well as achieve strength in the direction perpendicular to the bases such as substrates or supports (hereinafter, this direction will be also referred to as the “film thickness direction”).
Various studies and attempts have been made focusing on the shapes of phosphor crystals that form scintillator layers. For example, Patent Literature 1 is directed to the realization of radiographic conversion panels which may provide scintillator panels capable of giving X-ray images having high brightness and excellent sharpness. In detail, Patent Literature 1 discloses a radiographic conversion panel having on a substrate a phosphor layer which includes a phosphor base material in the form of columnar crystals with a specific shape. The phosphor layer in the radiographic conversion panel of Patent Literature 1 has a combination of a first phosphor layer with a specific film thickness including a phosphor base material and a second phosphor layer containing the phosphor base material and an activator. The inventors of Patent Literature 1 have found that excellent sharpness is obtained when the columnar phosphor crystals forming the phosphor layer satisfy a specific ratio of the crystal diameter at the outermost surface of the scintillator layer to the crystal diameter at 10 μm height from the substrate side.
Further, Patent Literature 2 describes an approach to enhancing the image quality (the brightness) of the obtainable radiographic images. In detail, Patent Literature 2 discloses that emission brightness is improved by forming a phosphor layer by combined use of a phosphor and an activator so as to obtain a uniform activator concentration throughout the phosphor layer. Other methods such as increasing the activator concentration on the X-ray incident side are also known.
Furthermore, Patent Literature 3 proposes a radiographic image conversion panel which includes a stack of stimulable phosphor layers configured such that lower intensity and higher intensity are emitted alternately from the stimulable phosphor layers when the layers are excited by the incidence of excitation light applied from an excitation light source to the radiographic image conversion panel in a direction from the support side toward the phosphor layer surface side. Patent Literature 4 proposes a radiographic image detector in which a scintillator has an activator concentration that repeatedly changes between high and low levels at least locally in a radiation propagation direction and the activator concentration at the upper and lower ends of columnar crystals is lower than the high levels of activator concentration.